Welcome to the Apple Support Communities
Snow Leopard is a paid upgrade. Buy Snow Leopard > http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard
Then, make a Time Machine backup and/or a clone with Carbon Copy Cloner, insert the DVD and upgrade.
After upgrading, open > Software Update to install the most recent version. Your Mac is compatible with OS X Mountain Lion, so if you want to upgrade to this version, open App Store and purchase Mountain Lion. The installer will download, so follow the steps to upgrade to Mountain Lion. Check if your programmes are supported > http://www.roaringapps.com